With the development of society, environmental pollution has become a restrict factor of economic development and is an important section in government legislation. Sulfur emission is severely restricted in developed countries. For example, according to the regulations of Environmental Protection Agency of The Federal Government of the United States, the SO2 emission in furnace flue gas, sulfur tail-gas and catalytic cracking regeneration flue gas in petroleum refining industry is limited to 50 ppm(v), equaling to about 143 mg/m3.
New techniques for tail-gas treatment of Claus sulfur recovery are under development. U.S. Pat. No. 7,311,891 B2 discloses a process for recovering sulfur from Claus tail-gas by adsorption, wherein the tail-gas comprising SO2 is passed through an adsorption bed wherein SO2 is adsorbed by an adsorbent. The SO2 is then deadsorbed by purging with an inert gas and the deadsorbed gas comprising SO2 is recycled to Claus apparatus for further conversion.
Main factors which influence the SO2 emission concentration of sulfur plant include purified tail-gas and waste-gas from degassing of liquid sulfur.
The total sulfur content in the purified tail-gas is closely related to the purification ability of desulphurizer and the conversion rate of the Claus tail-gas hydrogenation catalyst (particularly the content of organo-sulfur compound(s)). The purified tail-gas comprises mainly unadsorbed H2S and organo-sulfur compound(s), which are converted into SO2 through incineration in incinerator, leading to a SO2 emission of 150-500 mg/m3. The total sulfur content in the purified tail-gas can be decreased substantially by using a high efficiency desulphurizer. For example, compounded high efficiency desulphurizer HS103 from The Dow Chemical Company can decrease H2S in the purified tail-gas to not more than 10 ppm (10-20 mg/m3 SO2 emission).
Degassing of liquid sulfur is an important measure in the safety operation of sulfur recovery plant. Generally H2S is present in the sulfur produced by Claus process in an amount of 300-500 ppm(v). Without the degassing of the liquid sulfur, the polysulfide (H2SX) dissolved in the liquid sulfur will decompose to produce H2S during the storage, transportation and processing of liquid sulfur and the produced H2S will be released together with the H2S dissolved in the liquid sulfur. When H2S accumulates to a certain concentration, it will cause toxic action or even has a possibility of explosion. In another aspect, solid sulfur formed from undegassed sulfur is fragile, producing more fine particles and powder of sulfur during loading and unloading and transportation.
The basic principle of degassing of liquid sulfur is to release the dissolved H2S and to decompose polysulfide rapidly according to H2SX→H2S+SX−1, and finally remove H2S from liquid sulfur. The total H2S in the degassed liquid sulfur is not more than 0.001 wt %. Generally, the stripping gas for liquid sulfur degassing is air, steam, Claus tail-gas or nitrogen. Steam can lead to corrosion of pipes and equipments easily. H2S contained in Claus tail-gas will equilibrate with free H2S in the liquid sulfur and thus H2S in the liquid sulfur could not be reduced to less than 10 ppm. Nitrogen has low oxygen content and thus is not favorable for the oxidation of sulfur-containing compounds. Therefore air is preferably used as the stripping gas for degassing of liquid sulfur. However, the amount of gas to be processed will be increased due to the use of an external gas-source, leading to enlargement of pipes and equipments.
Currently, the waste-gas from degassing of liquid sulfur is treated by incinerating in an incinerator, wherein the waste-gas comprising H2S, sulfur vapor and the like is introduced into the incinerator directly and the sulfur-containing substance is burned into SO2, leading to a SO2 emission of 150-200 mg/m3.
There is always a requirement for processes for reducing sulfur emission of flue gas of a sulfur plant.